DC-DC converters transfer power from a DC electrical input source to a load by transferring energy between windings of an isolation transformer. The DC output voltage delivered to the load is controlled by adjusting the timing of internal power switching elements (e.g., by controlling the converter switching frequency and/or the switch duty cycle and/or the phase of switches). As defined herein, the functions of a “DC-DC converter” comprise: a) isolation between the input source and the load; b) conversion of an input voltage to an output voltage; and c) regulation of the output voltage. DC-DC converters may be viewed as a subset of a broad class of switching power converters, referred to as “switching regulators,” which convert power from an input source to a load by processing energy through intermediate storage in reactive elements. As defined herein, the functions of a “Switching Regulator” comprise: a) conversion of an input voltage to an output voltage, and b) regulation of the output voltage. If the required output voltage is essentially a positive or negative integer (or rational) multiple of the input voltage, the conversion function may also be efficiently performed by a capacitive “Charge Pump,” which transfers energy by adding and subtracting charge from capacitors.
Vinciarelli et al, “Efficient Power Conversion” U.S. Pat. No. 5,786,992 disclose expanding the operating voltage range of isolated DC-DC converters by connecting their inputs and/or outputs in series.
Non-resonant full-bridge, half-bridge, and push-pull DC-to-DC transformer topologies are known. See e.g., Severns and Bloom, “Modern DC-to-DC Switchmode Power Conversion Circuits,” ISBN 0-442-21396-4, pp. 78-111. Series, parallel, and other resonant forms of switching power converters are also known. See e.g., Steigerwald, “A Comparison of Half-Bridge Resonant Converter Topologies,” IEEE Transactions on Power Electronics, Vol. 2, No. 2, April, 1988. Variable frequency, series resonant, half-bridge converters for operation from an input voltage source are described in Baker, “High Frequency Power Conversion With FET-Controlled Resonant Charge Transfer,” PCI Proceedings, April 1983, and in Nerone, U.S. Pat. No. 4,648,017. Half-bridge, single-stage, ZVS, multi-resonant, variable frequency converters, which operate from an input voltage source are shown in Tabisz et al, U.S. Pat. No. 4,841,220 and Tabisz et al, U.S. Pat. No. 4,860,184. A variable frequency, full-bridge, resonant converter, in which an inductor is interposed between the input source and the resonant converter is described in Divan, “Design Considerations for Very High Frequency Resonant Mode DC/DC Converters,” IEEE Transactions on Power Electronics, Vol. PE-2, No. 1, January, 1987. A variable frequency, ZVS, half-bridge LLC series resonant converter is described in Bo Yang et al, “LLC Resonant Converter for Front End DC-DC Conversion,” CPES Seminar 2001, Blacksburg, Va., April 2001. Analysis and simulation of a “Low Q” half-bridge series resonant converter, wherein the term “Low Q” refers to operation at light load, is described in Bo Yang et al, “Low Q Characteristic of Series Resonant Converter and Its Application,” CPES Seminar 2001, Blacksburg, Va., April 2001.
Fixed-frequency half-bridge and full-bridge resonant converters are also known in which output voltage control is achieved by controlling the relative timing of switches. A half-bridge, single-stage, ZVS, multi-resonant, fixed-frequency converter that operates from an input voltage source is shown in Jovanovic et al, U.S. Pat. No., 4,931,716. A full-bridge, single-stage, ZVS, resonant, fixed-frequency converter that operates from an input voltage source is shown in Henze et al, U.S. Pat. No. 4,855,888.
A full-bridge, single-stage, ZCS, series-resonant, fixed-frequency converter, operating at a frequency equal to the characteristic resonant frequency of the converter, is shown in Palz, “Stromversorgung von Satelliten-Wanderfeldröhren hoher Leistung” (“Power Supply for Satellitesm—High Capacity Traveling—Wave Tubes”), Siemens Zeitschrift, Vol. 48, 1974, pp. 840-846. Half and full-bridge, single-stage, ZVS, resonant, converters, for powering fluorescent tubes are shown in Nalbant, U.S. Pat. No. 5,615,093.
A DC-to-DC Transformer offered for sale by SynQor, Hudson, Mass., USA, called a “BusQor™ Bus Converter,” that converts a regulated 48 VDC input to a 12 VDC output at a power level of 240 Watts and that can be paralleled with other similar converters for increased output power delivery, and that is packaged in a quarter brick format, is described in data sheet “Preliminary Tech Spec, Narrow Input, Isolated DC/DC Bus Converter,” SynQor Document No. 005-2BQ512J, Rev. 7, August, 2002.
The art of resonant power conversion, including operation below or above resonant frequency, utilizing either ZCS or ZVS control techniques and allowing the resonant cycle to be either completed or purposely interrupted, is summarized in Chapter 19 of Erickson and Maksimovic, “Fundamentals of Power Electronics,” 2nd Edition, Kluwer Academic Publishers, 2001.
Cascaded converters, in which a first converter is controlled to generate a voltage or current, which serves as the source of input power for a DC-to-DC transformer stage, are known. A discussion of canonical forms of cascaded converters is given in Severns and Bloom, ibid, at, e.g., pp. 114-117, 136-139. Baker, ibid, discusses the use of a voltage pre-regulator cascaded with a half-bridge, resonant, variable-frequency converter. Jones, U.S. Pat. No. 4,533,986 shows a continuous-mode PWM boost converter cascaded with both PWM converters and FM resonant half-bridge converters for improving holdup time and improving the power factor presented to an AC input source. A zero-voltage transition, current-fed, full-bridge PWM converter, comprising a PWM boost converter delivering a controlled current to a PWM, full-bridge converter, is shown in Hua et al, “Novel Zero-Voltage Transition PWM Converters,” IEEE Transactions on Power Electronics, Vol. 9, No. 2, March, 1994, p. 605. Stuart, U.S. Pat. No. 4,853,832, shows a full-bridge series-resonant  converter cascaded with a series-resonant DC-to-DC transformer stage for providing AC bus power to distributed rectified loads. A half-bridge PWM DC-to-DC transformer stage for use in providing input power to point-of-load DC-DC converters in a DPA is described in Mweene et al, “A High-Efficiency 1.5 kW, 390-50V Half-Bridge Converter Operated at 100% Duty Ratio,” APEC '92 Conference Proceedings, 1992, pp. 723-730. Schlecht, U.S. Pat. Nos. 5,999,417 and 6,222,742 shows DC-DC converters which incorporate a DC-to-DC transformer stage cascaded with a switching regulator. Vinciarelli, “Buck-Boost DC-DC Switching Power Conversion,” U.S. patent application Ser. No. 10/214,859, filed Aug. 8, 2002, assigned to the same assignee as this application and incorporated by reference, discloses a new, high efficiency, ZVS buck-boost converter topology and shows a front-end converter comprising the disclosed topology cascaded with a DC-DC converter and a DC-to-DC transformer.
In one aspect, prior art approaches to off-line power conversion may be characterized by how they accommodate a broad range of nominal line voltages, e.g., 110 VAC (i.e. 85-120 VAC) and 220 AC (i.e. 170-240 VAC). In one approach, the line is simply rectified and power conversion circuitry is designed to operate over the full range of variation of the rectified line voltage; in another approach, called “auto-ranging”, the rectification circuitry is reconfigured based upon the nominal value of the line voltage and the range of voltages over which power conversion circuitry must operate is reduced. In another aspect, off-line power conversion may be characterized in terms of whether or not power factor correction (“PFC”) is provided. Auto ranging is commonly provided in non-PFC power supplies using a capacitive voltage doubler. Referring to FIG. 10 for example, an off-line power supply includes a bridge rectifier 501, capacitors 502 and 503 connected in series across the rectifier output, and a doubler switch 506 which may be manually or automatically controlled for effecting voltage doubling. For high line voltages e.g. 220 VAC the switch remains open and the rectified voltage V2 will approximately equal the peak input voltage VIN. For low line applications, the switch 506 is closed and V2 will approximately equal twice the peak input voltage VIN and the voltage V2 will remain nominally at 220V regardless of whether a 110 or 220 VAC line is connected at the input. The DC-DC converter 504 provides the voltage transformation, isolation and regulation functions for power delivered to the load 505.
Because it requires the use of energy storage capacitors at the output of the rectifiers, the capacitive voltage-doubler is not generally suitable for use in PFC applications. Vinciarelli et al., “Passive Control of Harmonic Current Drawn From an AC Input by Rectification Circuitry,” U.S. Pat. No. 6,608,770, issued Aug. 19, 2003, assigned to the same assignee as this application and incorporated by reference, discloses capacitive voltage-doubling auto-ranging circuitry which passively controls the harmonic current drawn from an AC line.
Another auto-ranging approach suitable for both PFC and non-PFC applications is the use of a line frequency transformer with switched windings. The line voltage may be applied across all or part of the primary winding depending on the applied line voltage. In PFC applications the more common approach is use of a PFC boost converter as shown in FIG. 11. The off-line auto-ranging PFC power supply of FIG. 11 includes bridge rectifier 501, non-isolated PFC Boost converter 507, and storage capacitor 508, followed by isolated DC-DC converter 504. In order to control the current drawn from the AC line for PFC, the output voltage VB of the boost converter must be set to a voltage greater than the highest peak input voltage VIN. In a typical power supply designed for international use, the boost voltage may be 400V. Power is then converted from the boost voltage down to the load voltage by DC-DC converter 504, which provides voltage transformation, regulation, and isolation. Operation of the boost and DC-DC converters at such high voltages includes cost and performance penalties including, lower figure of merit for switches at high voltages and safety issues for energy storage at high voltages.
One solution, disclosed in Vinciarelli et al., “Efficient Power Conversion,” U.S. Pat. No. 5,786,992, issued Jul. 28, 1998, assigned to the same assignee as this application and incorporated by reference, configures power converters in series and parallel allowing the combination of converters to operate over a greater voltage range.